This invention relates to a circuit for controlling the bias voltage on an X-ray or other type of vacuum tube to provide for the tube conducting low current when there is a high voltage drop between its anode and cathode and high current when there is a low voltage drop between its anode and cathode.
The new bias control was developed primarily for solving the problems that arise in connection with switching an X-ray tube between high energy and low energy output states as is required in digital fluorography, particularly hybrid digital subtraction fluorography (DSF).
One hybrid DSF method requires projecting low and high energy X-ray beam pulses of several millisecond durations alternately through a patient. It is desirable for the pulses to be separated by no more than two television frame times. There may be 50 to 80 high and low energy pulse pairs produced in a typical X-ray exposure sequence extending over several seconds. By way of example, and not limitation, the peak kilovoltage applied to the anode of the X-ray tube may be around 135 kilovolts for the high energy exposures and the X-ray tube current may be on the order of 100 milliamperes (mA). For the low energy exposure pulses, the peak anode voltage may be on the order of 70 kilovolts and X-ray tube current may be as high as 1000 mA. Usually, the individual X-ray pulses will be delivered within a single television frame time which is typically 1/30 or 1/25 of a second.
The terms low X-ray energy and high X-ray energy are used for convenience. It would be more accurate to say that they are low and high average energy X-ray pulses. This is for the well-known reason that even when an absolutely constant voltage is applied to the anode of an X-ray tube some of the output X-ray photons will have peak energy while others will have lower energy. In other words, there is a spectral distribution of energies within particular low and high energy limits.
Generally, an X-ray image intensifier is used to convert the different energy X-ray images to optical images which are viewed by a television camera. The analog video signal frames are converted to digital picture elements (pixels) for further processing in accordance with the requirements of digital subtraction fluorography. One use of DSF is, of course, to provide the physician with an image of the interior of blood vessels in a region of interest within the patient's body. Visualization is enhanced by making some exposures subsequent to the time an X-ray contrast medium, such as an iodinated compound that has been injected into the circulatory system, arrives at and flows through the vessels that are the subject of the arteriographic examination. Post-contrast arrival images are then subtracted from pre-contrast images to produce a sequence of difference images in which soft tissue and bone are subtracted out while the contrast medium remains to enable visualization of the interior outline of the vessel.
In one hybrid digital subtraction fluorography mode, a sequence of rapidly occurring low and high energy exposures are made continuously through the pre-contrast interval, the post-contrast interval and an after-post-contrast interval. The first low energy exposure or image is retained in a memory as a mask. Similarly, the first high energy exposure image is stored in a memory as a mask. Then all of the subsequent low energy images in the sequence are subtracted from the mask and the resulting series of difference images are converted to analog video format and stored on video disk. The alternate subsequent high energy images are subtracted from the high energy mask and stored on disk. Subtracting images or exposures made at identical energy levels with a substantial amount of time between them is called temporal subtraction. This type of subtraction cancels everything that is unchanged in the respective images. For instance, ordinarily bone and soft tissue attenuation will be unchanged from image to image but projected intensity of the contrast medium will not be so substantially everything but the contrast medium will subtract out or cancel. If there is substantial movement of the patient's tissue such as due to peristalsis or coughing in the course of a temporal subtraction procedure, there will be motion artifacts in the subtracted images which will not cancel. Noise and motion artifacts may be eliminated by resorting to hybrid subtraction.
For hybrid subtraction, all of the low energy temporal difference images are summed. Similarly, all of the high energy temporal difference images are summed. Then the results of the two summations are subtracted to produce a final difference image in which soft tissue and bone and anything else that remains constant is cancelled out while the contrast medium that defines the blood vessel remains.
In any case, it is desirable to be able to produce the low and high X-ray energy pulses in a pair rapidly and as close together as possible so there can be no substantial involuntary movement of the patient between a low energy pulse and the next ensuing high energy pulse.
Besides a hybrid subtraction requiring accurate timing of the X-ray pulses, it is important to apply the identical kilovoltage and have the same X-ray tube current for every low and high energy exposure in a sequence. It is also necessary for the X-ray tube current or mA to be low for high kilovoltage and for the mA to be high for low kilovoltage so that the intensities of the photons that emerge from the body are substantially identical for the low and high energy exposures.
The bias voltage applied to the grid of an X-ray tube can be reduced to zero volts for the low energy or low kilovoltage pulses, allowing full mA, and a more negative bias voltage can be applied during the alternate high kilovoltage pulses, allowing reduced mA to maintain approximately constant wattage from pulse to pulse at each energy. There are several known X-ray tube grid bias control systems. They usually employ a transformer that is in an oil-filled tank for producing an alternating voltage that is rectified and switched from pulse to pulse to obtain zero bias voltage for the low energy exposures and, by way of example and not limitation, -3000 volts dc for the low current, high kilovoltage or high energy exposures. The size of the bias equipment and the insulating requirements for isolating the bias circuits from high kilovoltage circuits up to about 150 kilovolts for the X-ray tube anode results in equipment that is costly, voluminous and subject to failure, especially in the switching circuit.
The prior art circuits do not allow for selectability or fine tuning of the different bias voltages. They do not permit free choice of X-ray tube current and tube kilovoltage combinations. For instance, there are occasions where the body part being fluorographed requires different low and high energy X-ray tube currents and voltages than other parts of the body in order to get the best images for subtraction.